A polymer electrolyte fuel cell in which a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas are supplied to an anode and a cathode, respectively, and an electromotive force is generated by an electrochemical reaction occurring at the electrodes is generally constituted by laminating a bipolar plate, a gas diffusion electrode substrate, a catalyst layer, an electrolyte membrane, a catalyst layer, a gas diffusion electrode substrate and a bipolar plate in this order. The gas diffusion electrode substrate is required to have high gas diffusivity for allowing a gas supplied from the bipolar plate to be diffused into a catalyst layer and high water removal performance for discharging water generated by the electrochemical reaction to the bipolar plate, as well as high electrical conductivity for extracting generated electric current. Thus, gas diffusion electrode substrates are widely used in which a microporous layer is formed on a surface of a substrate that is a carbon sheet composed of a carbon fiber etc.
However, such gas diffusion electrode substrates have the following problems: (1) when the polymer electrolyte fuel cell is operated at a relatively low temperature of below 70° C. in a high current density region, as a result of blockage of the gas diffusion electrode substrate by water generated in a large amount and shortage in the gas supply, the fuel cell performance is impaired (this problem is hereinafter referred to as “flooding”); and (2) the carbon sheet that forms the gas diffusion electrode substrate is delaminated in the thickness direction in a production process of the fuel cell, which causes a problem when the carbon sheet is conveyed, or generated water is retained in the delaminated part of the carbon sheet during electrical power generation, so that the fuel cell performance is impaired. Thus, the gas diffusion electrode substrate is desired to have mechanical strength required as the gas diffusion electrode substrate while exhibiting high generated water removal performance and delamination resistance.
There has been proposed a fuel cell gas diffusion electrode substrate capable of preventing deterioration of the fuel cell performance owing to the resin density of the carbon sheet which is continuously decreased in the thickness direction (Patent Document 1).
There has also been proposed a gas diffusion electrode substrate capable of preventing deterioration of the fuel cell performance without having a microporous layer, owing to a high-density region in a region of 40 μm in thickness from the surface of the carbon sheet, and a low-density region in the middle of the carbon sheet (Patent Document 2).
In addition, there has been proposed a carbon sheet capable of preventing deterioration of performance of the fuel cell, the carbon sheet including a laminate of two sheets different in resin density and having different pore diameters in the thickness direction (Patent Document 3).